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If you are caught taking drugs then you are by definition caught in possession of a controlled substance.
If you are arrested for being high when not in possession the police would have to prove that you were, at some point, in possession of the substance that got you high. I'm sure they've got bigger fish to fry. If they try to prosecute you for the act of being high then it's pretty easy to say something along the lines of; my drink was spiked / someone gave me a cookie to try it tasted a bit funny.
IANAL
Would appreciate a lawyer"s input on that because the misuse of drug act does not specify taking drugs as a crime. I wonder which law it is a crime under?